Transparent conductors are widely used in the flat-panel display industry to form electrodes that are used to electrically switch light-emitting or light-transmitting properties of a display pixel, for example, in liquid crystal or organic light-emitting diode displays. Transparent conductive electrodes are also used in touch screens in conjunction with displays. In such applications, the transparency and conductivity of the transparent electrodes are important attributes so that they do not inhibit the visibility of the displays. In general, it is desired that transparent conductors have a high transparency (for example, greater than 90% in the visible spectrum) and a low electrical resistivity (for example, less than 10 ohms/square).
Touch screens with transparent electrodes are widely used with electronic displays, especially for mobile electronic devices. Such devices typically include a touch screen mounted over an electronic display that displays interactive information. Touch screens mounted over a display device are largely transparent so a user can view displayed information through the touch-screen and readily locate a point on the touch-screen to touch and thereby indicate the information relevant to the touch. By physically touching, or nearly touching, the touch screen in a location associated with particular information, a user can indicate an interest, selection, or desired manipulation of the associated particular information. The touch screen detects the touch and then electronically interacts with a processor to indicate the touch and touch location. The processor can then associate the touch and touch location with displayed information to execute a programmed task associated with the information. For example, graphic elements in a computer-driven graphic user interface are selected or manipulated with a touch screen mounted on a display that displays the graphic user interface. Referring to FIG. 20, a prior-art display and touch-screen apparatus 100 includes a display 110 with a corresponding touch screen 120 mounted with the display 110 so that information displayed on the display 110 is viewed through the touch screen 120. Graphic elements displayed on the display 110 are selected, indicated, or manipulated by touching a corresponding location on the touch screen 120. The touch screen 120 includes a first transparent substrate 122 with first transparent electrodes 130 formed in the x dimension on the first transparent substrate 122 and a second transparent substrate 126 with second transparent electrodes 132 formed in the y-dimension facing the x-dimension first transparent electrodes 130 on the second transparent substrate 126. A dielectric layer 124 is located between the first and second transparent substrates 122, 126 and first and second transparent electrodes 130, 132. Referring also to the plan view of FIG. 21, in this example first pad areas 128 in the first transparent electrodes 130 are located adjacent to second pad areas 129 in the second transparent electrodes 132. (The first and second pad areas 128, 129 are separated into different parallel planes by the dielectric layer 124.) The first and second transparent electrodes 130, 132 have a variable width and extend in orthogonal directions (for example as shown in U.S. Patent Application Publication Nos. 2011/0289771 and 2011/0099805). When a voltage is applied across the first and second transparent electrodes 130, 132, electric fields are formed between the first pad areas 128 of the x-dimension first transparent electrodes 130 and the second pad areas 129 of the y-dimension second transparent electrodes 132.
A display controller 142 (FIG. 20) connected through electrical buss connections 136 controls the display 110 in cooperation with a touch-screen controller 140. The touch-screen controller 140 is connected through electrical buss connections 136 and wires 134 and controls the touch screen 120. The touch-screen controller 140 detects touches on the touch screen 120 by sequentially electrically energizing and testing the x-dimension first and y-dimension second transparent electrodes 130, 132.
Referring to FIG. 22, in another prior-art embodiment, rectangular first and second transparent electrodes 130, 132 are arranged orthogonally on first and second transparent substrates 122, 126 with intervening dielectric layer 124, forming touch screen 120 which, in combination with the display 110 forms the touch screen 120 and display apparatus 100. In this embodiment, first and second pad areas 128, 129 coincide and are formed where the first and second transparent electrodes 130, 132 overlap. The touch screen 120 and display 110 are controlled by touch screen and display controllers 140, 142, respectively, through electrical buss connections 136 and wires 134.
Since touch-screens are largely transparent so as not to inhibit the visibility of the displays over which the touch-screens are located, any electrically conductive materials located in the transparent portion of the touch-screen either employ transparent conductive materials or employ conductive elements that are too small to be readily resolved by the eye of a touch-screen user. Transparent conductive metal oxides are well known in the display and touch-screen industries and have a number of disadvantages, including limited transparency and conductivity and a tendency to crack under mechanical or environmental stress. This is particularly problematic for flexible touch-screen-and-display systems. Typical prior-art conductive electrode materials include conductive metal oxides such as indium tin oxide (ITO) or very thin layers of metal, for example silver or aluminum or metal alloys including silver or aluminum. These materials are coated, for example, by sputtering or vapor deposition, and are patterned on display or touch-screen substrates, such as glass. However, the current-carrying capacity of such electrodes is limited, thereby limiting the amount of power that can be supplied to the pixel elements. Moreover, the substrate materials are limited by the electrode material deposition process (e.g. sputtering). Thicker layers of metal oxides or metals increase conductivity but reduce the transparency of the electrodes.
Various methods of improving the conductivity of transparent conductors are taught in the prior art. For example, U.S. Pat. No. 6,812,637 describes an auxiliary electrode to improve the conductivity of the transparent electrode and enhance the current distribution. Such auxiliary electrodes are typically provided in areas that do not block light emission, e.g., as part of a black-matrix structure.
It is also known in the prior art to form conductive traces using nano-particles including, for example silver. The synthesis of such metallic nano-crystals is known. For example, U.S. Pat. No. 6,645,444 describes a process for forming metal nano-crystals optionally doped or alloyed with other metals. U.S. Patent Application Publication No. 2006/0057502 describes fine wirings made by drying a coated metal dispersion colloid into a metal-suspension film on a substrate, pattern-wise irradiating the metal-suspension film with a laser beam to aggregate metal nano-particles into larger conductive grains, removing non-irradiated metal nano-particles, and forming metallic wiring patterns from the conductive grains. However, such wires are not transparent and thus the number and size of the wires limits the substrate transparency as the overall conductivity of the wires increases.
Touch-screens including very fine patterns of conductive elements, such as metal wires or conductive traces are known. For example, U.S. Patent Application Publication No. 2011/0007011 teaches a capacitive touch screen with a mesh electrode, as does U.S. Patent Application Publication No. 2010/0026664. Referring to FIG. 23, a prior-art x- or y-dimension variable-width first or second transparent electrode 130, 132 includes a micro-pattern 156 of micro-wires 150 arranged in a rectangular grid or mesh. The micro-wires 150 are multiple very thin metal conductive traces or wires formed on the first and second transparent substrates 122, 126 (not shown in FIG. 23) to form the x- or y-dimension first or second transparent electrodes 130, 132. The micro-wires 150 are so narrow that they are not readily visible to a human observer, for example 1 to 10 microns wide. The micro-wires 150 are typically opaque and spaced apart, for example by 50 to 500 microns, so that the first or second transparent electrodes 130, 132 appear to be transparent and the micro-wires 150 are not distinguished by an observer.
It is known that micro-wire electrodes in a touch-screen can visibly interact with pixels in a display and various layout designs are proposed to avoid such visible interaction. Thus, the pattern of micro-wires in a transparent electrode is important for optical as well as electrical reasons.
A variety of layout patterns are known for micro-wires used in transparent electrodes. U.S. Patent Application Publication 2010/0302201, U.S. Patent Application Publication No. 2012/0031746, U.S. Patent Application Publication No. 2012/0162116, and U.S. Patent Application Publication No. 2011/0291966 teach various micro-wire patterns used for electrodes in capacitive touch screens. For example, FIG. 24 illustrates first and second orthogonal transparent electrodes 130, 132 having a micro-pattern 156 of micro-wires 150 arranged in a diamond pattern.
When in operation, electronic circuits such as those used to control arrays of pixels in a flat-screen display or to drive electrodes in a capacitive touch screen emit electromagnetic radiation that interferes with other nearby, electronic devices. For example, the signal lines and transistors that control the behavior of pixels in a flat-screen display emit electromagnetic radiation that can interfere with signals in a nearby touch screen. Likewise, the electrodes that are controlled to sense capacitance in a capacitive touch screen emit electromagnetic radiation that can interfere with signal lines and transistors in a nearby flat-screen display. Since touch screens and display devices are typically laminated together in a thin package, such interference can reduce the signal transmission rate or cause spurious signals in either or both of a laminated touch screen and display device.
There is a need, therefore, for an improved method and structure for reducing electromagnetic interference in a touch-screen-and-display apparatus that does not reduce visibility of the display, is robust in the presence of mechanical stress, and reduces reflections.